XHTML vs HTML

In XHTML, in contrast to HTML, all tags have to be closed.
The advantage of using XHTML is that it makes your source code clean and readable. And that makes your webpages look professional.

Also, XHTML is more compatible with other languages like XML. But that's not a direct advantage.

Writing clean code is very important.
Although all browsers know how to deal with correct HTML, they tend to act different if the HTML code is incorrect.
Most browsers try to repair that 'on the fly', but the results may differ a lot.
Also, search engine spider bots have an easier task reading correct HTML, otherwise, like the browser, they have to repair the page first and then results will become unpredictable.

That's why it is very wise to use XHMTL (because of the tidy coding) and to verify your code.
One great place to do that is W3.org. Enter the URL of your webpage and they will tell you if your XHMTL code is ok or else, what's is wrong with it.

Another great and faster resource is the HTML Validator Plugin for Firefox browsers.